FYI, I launched the same rocket again this past weekend, same chute and same JLCR, but this time with a bigger motor (K360), flew to 6111'. Once again, the rocket was dropping really fast until JLCR released the chute at 500' for a perfect landing (see graph below). I'm still thinking about adding a drogue but it may not be necessary.

Looks like it was falling really fast from apogee until about 1800 feet and then slowed down but you also had some event at 3000'. Are you sure it even separated at apogee? Looks like 3 different descent rate. Any backup altimeter data?

I noticed that as well, sure looks like something happened at 1800'. Maybe the chute slipped partially out of the JLCR. My recorder was off-rocket at apogee, but on the video right after the Telemetrum announcer says "Drogue," Brian and my son both exclaimed, "I see it!" Then Brian said, "That looks good, man." So I assume there was a deployment. See https://youtu.be/eSMdiALF9nE.

Looks like it was falling really fast from apogee until about 1800 feet and then slowed down but you also had some event at 3000'. Are you sure it even separated at apogee? Looks like 3 different descent rate. Any backup altimeter data?

billdz has moved recovery into the next dimension - not single deploy, not dual deploy, but triple deploy!

I am still struggling with the fact that this 4", 3000g rocket is falling drogueless at 150 ft/s. What is the configuration of the rocket pieces as it is falling before the JLCR releases? Are the two pieces of similar mass and drag? Flat spin/tumble? Nose heavy? I would think that nose down semi-ballistic is the only way this thing can fall that fast.

Also, the 5 sec lag for the green velocity curve to catch up with the slope of the red altitude curve is puzzling. My SLCF altimeters don't do that, as the terminal velocity occurs very quickly, and the SLCF alt and vel curves are in sync. I wonder if this Telemetrum data is over-filtered in some way.

Looks like it was falling really fast from apogee until about 1800 feet and then slowed down but you also had some event at 3000'. Are you sure it even separated at apogee? Looks like 3 different descent rate. Any backup altimeter data?

since that was accelerometer data, I suspect it was the two parts of the rocket hitting together that cause the big spike since nothing else seemed to change. Obviously a strong collision.

"Did you have motor backup? Two altimeters with 2nd apogee charge coming in late?" This flight was motor ejection only, no charges.

"Maybe perfect landing something caused 3 different descent rates. Maybe main packed too tight?" From the graph it sure looks like something happened at 1800'. Based on the video I don't think the chute was stuck in the airframe until the rocket was at 1800'. I'll ask Brian if he recalls anything. And see post #30 in this thread, where John Beans said, "The velocity graph isn't useful much past burnout."

"Did you have motor backup? Two altimeters with 2nd apogee charge coming in late?" This flight was motor ejection only, no charges.

"Maybe perfect landing something caused 3 different descent rates. Maybe main packed too tight?" From the graph it sure looks like something happened at 1800'. Based on the video I don't think the chute was stuck in the airframe until the rocket was at 1800'. I'll ask Brian if he recalls anything. And see post #30 in this thread, where John Beans said, "The velocity graph isn't useful much past burnout."

OK Bill, it was a perfect landing and the acell data is crap after apogee but that doesn't explain why you have 3 clearly different descent rates. Seems like you are avoiding answering some questions.

I spoke with Brian, who had eyes on the rocket all the way down. He says he has no recollection of the configuration during descent and he recalls nothing that seemed abnormal.
@Steve: The Telemetrum was in the NC but otherwise no weight added. Not sure what you mean by "stability margin after burnout." Stability at liftoff was 2.9 cal.
@Tim: What are you talking about? What questions do you think I'm avoiding? I've reported everything I know. To everyone who was there, this appeared to be a nice, problem-free flight. I did not know about the third descent rate until I saw the graph.

Descent: How fast is too fast before main chute deploys?

Originally Posted by billdz

I spoke with Brian, who had eyes on the rocket all the way down. He says he has no recollection of the configuration during descent and he recalls nothing that seemed abnormal.
@Steve: The Telemetrum was in the NC but otherwise no weight added. Not sure what you mean by "stability margin after burnout." Stability at liftoff was 2.9 cal.
@Tim: What are you talking about? What questions do you think I'm avoiding? I've reported everything I know. To everyone who was there, this appeared to be a nice, problem-free flight. I did not know about the third descent rate until I saw the graph.

Your stability margin will increase as the motor burns, decreasing in mass and shifting the Cg forward. A margin of 2.9 is almost twice as large as needed, so the rocket was already somewhat over-stable. The stability margin would have increased until burnout, when itís at its max. Over-stability is not what you want with a drogueless rocket. An empty nosecone tends to float in the air, helping to destabilize the rocket during descent and resulting in a slower descent.
Iíve seen lots of rockets that had multiple descent rates. Sometimes itís as simple as shifting from a flat spin to a nose down. I wasnít there and so my guesses are just that but with some experience, both first hand and observed. Iím just trying to help you understand what might have happened.

Steve Shannon

Last edited by Steve Shannon; 7th December 2017 at 03:00 AM.
Reason: I said Cg shifts rearward when engine burns, but I meant forward.

Thanks, Steve. I'm aware that stability increases as the propellant burns away, that an over stable rocket is prone to weathercocking, and that 2.9 is on the high side of what we want. I was not aware that stability is a factor during descent. In my case, if the empty NC caused a slower descent, that would be a good thing, wouldn't it? The original question was whether the rocket was falling too quickly. I hope that the multiple descent rate was caused simply by a shifting of the position of the components, that seems logical.

@Tim: What are you talking about? What questions do you think I'm avoiding? I've reported everything I know. To everyone who was there, this appeared to be a nice, problem-free flight. I did not know about the third descent rate until I saw the graph.

Thanks, Steve. I'm aware that stability increases as the propellant burns away, that an over stable rocket is prone to weathercocking, and that 2.9 is on the high side of what we want. I was not aware that stability is a factor during descent. In my case, if the empty NC caused a slower descent, that would be a good thing, wouldn't it? The original question was whether the rocket was falling too quickly. I hope that the multiple descent rate was caused simply by a shifting of the position of the components, that seems logical.

Yes, it would be a good thing. An empty nosecone, or a drogue chute, can help by allowing your rocket to fall sideways rather than forward down. Too much stability, even with the nosecone off, can cause the rocket to fall nose down, which is almost certainly going to be faster than falling sideways or in a flat spin. At some speed, depending on how you fold your chute, the jerk experienced when the chute opens can do damage to either the chute or the rocket. Every rocket is different, but knowing the margin of stability after the motor is spent adds to your understanding. You should be able to use either RockSim or OpenRocket to calculate it.

Seems like 150 fps would be really hard to do unless the rocket was streamlined somehow. I mean, on the one hand it exhibits classic flutter in the pressure data (all the bumpy stuff) like it would on tumble, on the other hand it's moving 150 fps, which is practically lawn-dart speed.
Maybe it found some really stable streamlined configuration on the way down that got disrupted (somehow) at 1800 ft, then it slowed to a more reasonable tumble rate? Dunno.

With regard to configuration, someone took this pic at last week's launch, it shows the same rocket descending before the JLCR released the chute. Speed was 80 fps. The chute is yellow and there's a black nomex protector just to the right of the chute.

With regard to configuration, someone took this pic at last week's launch, it shows the same rocket descending before the JLCR released the chute. Speed was 80 fps. The chute is yellow and there's a black nomex protector just to the right of the chute.

The pic is not from the same flight we have been discussing, it is from a recent flight with an I165 motor, apogee was just 1730'. Descent speed was 50-80 fps from apogee until JLCR released the chute. This is about the same descent speed as it was for the earlier 6111' flight after the event at 1800'. So probably John is right, "it found some really stable streamlined configuration on the way down that got disrupted (somehow) at 1800 ft". Would be great to know the configuration before the rocket slowed down at 1800', but guess we will never know.